My son spoke to the ref after his last scrimmage to get some clarification on the arm bar rule from a ref's POV. His take away was that the defensive players are going to take a lot of hits to the gut without the arm bar "protection." What was the reason for the change?

The arm bar has always been illegal.

The actual rule change this year regarding hand checking is because after years of this being a point of emphasis, officials have let it get out of hand across the country leading to an increased amount of rough play.